The redox potential of natural fresh waters measured with a platinum electrode, using a standard hydrogen electrode as reference, usually ranges between 0.4 and 0.6 volts. The bottom sediment in natural water bodies is often fully reduced having a potential near zero, but on contact with oxygen-rich water the top layer of the mud (a few mm) becomes oxidized. This is referred to as the oxidized microzone layer of the bottom sediment, which can have a redox potential of about 0.2 volts.
The oxidized microzone has a higher capacity to absorb ions. The diffusion of ferrous and phosphate ions into the oxygen-rich microzone leads to its precipitation as ferric phospnate and thus the oxidized microzone layer prevents the diffusion of these ions from the mud to water. But with overlying anoxic waters this barrier collapses when the microzone becomes reduced. These changes can cause profound effect in water chemistry and biology of the fish pond, which are yet to be fully elucidated.